1. The Technical Field
This invention relates generally to the art of the microwave heating by high frequency electromagnetic radiation or microwave energy. More particularly, the present invention relates to ceramic compositions useful for fabrication into microwave susceptors, and to microwave heating susceptors fabricated therefrom, suitable for disposable microwave packages for food products
2. Background Art
The heating of food articles with microwave energy by consumers has now become commonplace. Such microwave heating provides the advantages of speed and convenience. However, heating certain food items, e.g., breaded fish portions with microwaves often gives them a soggy texture and fails to impart the desirable browning flavor and/or crispness of conventionally oven heated products due in part to retention of oil and moisture. Unfortunately, if microwave heating is continued in an attempt to obtain a crisp exterior, the interior is generally overheated or overdone.
The prior art includes many attempts to overcome such disadvantages while attempting to retain the advantages of microwave heating. That is, the prior art includes attempts at providing browning or searing means in addition to microwave heating. Basically, three approaches exist whether employing permanent dishes or disposable packages to providing microwave heating elements which provide such browning or searing and which elements are referred to herein and sometimes in the art as microwave heating susceptors. In the art, materials which are microwave absorptive are referred to as "lossy" while materials which are not are referred to as "non-lossy" or, equivalently, merely "transparent."
The first approach is to include an electrically resistive film usually quite thin, e.g., 0.00001 to 0.00002 cm., applied to the surface of a non-conductor or non-lossy substrate. In the case of a permanent dish, the container is frequently ceramic while for a disposable package the substrate can be a polyester film. Heat is produced because of the I.sup.2 R or resistive loss (see, for example, U.S. Pat. Nos. 3,853,612, 3,705,054, 3,922,452 and 3,783,220). Examples of disposable packaging materials include metallized films such as described in U.S. Pat. Nos. 4,594,492, 4,592,914, 4,590,349, 4,267,420 and 4,230,924.
A second category of microwave absorbing materials comprise electric conductors such as parallel rods, cups or strips which function to produce an intense fringing electric field pattern that causes surface heating in an adjacent food. Examples include U.S. Pat. Nos. 2,540,036, 3,271,552, 3,591,751, 3,857,009, 3,946,187 and 3,946,188. Such an approach is usually taken with reusable utensils or dishes.
A third approach is to form articles from a mass or bed of particles that become hot in bulk when exposed to microwave energy. The microwave absorbing substance can be composed of ferrites, carbon particles, etc. Examples of such compositions or articles prepared therefrom include, U.S. Pat. Nos. 2,582,174, 2,830,162 and 4,190,757. These materials can readily experience runaway heating and immediately go to temperatures in excess of 1200.degree. F. Some control over final heating temperature is obtained by lowering of Curie point by addition of dopants or selected binders.
A review of the prior art, especially that art directed towards provision of heating susceptors for disposable packages for microwave heating of foods indicates that at least three basic problems exist in the formulation and fabrication of heating susceptors. One difficulty with the third category of materials, generally, is that they can exhibit runaway heating, that is, upon further microwave heating their temperature continues to increase. Great care must be taken in fabrication of safe articles containing such materials. Metallized film materials of the first category can be formulated and fabricated such that they do not exhibit runaway heating. However, such films suffer from the second problem; namely that while their operating temperatures are quite hot, are at controlled temperatures, and are sufficient to brown the surface of nearby food items, due to their thinness and low mass, only small quantities of heat are actually generated. Such materials are thus unsuitable for certain foods which require absorption of great amounts of heat or "deep heating" in their preparation, e.g., cake batters. The third general problem is one of cost. Microwave susceptors frequently comprise costly materials. Also, fabrication of susceptor structures frequently is complex and expensive.
Accordingly, in view of the above-noted problems with present microwave susceptors, an object of the present invention is to provide materials and devices fabricated therefrom which will heat under the influence of the microwave radiation up to an upper temperature limit at which temperatures the devices come to an equilibrium and cease substantially to absorb additional microwave energy and heating to a higher temperature is precluded.
Another object of the present invention is to provide heating materials for and devices fabricated therefrom which are disposable and adapted for use with preprepared foods.
A still further object of the present invention is to provide heating materials for and devices fabricated therefrom which can be utilized as a non-disposable utensil.
A still further object of the present invention is to provide heating materials for and devices fabricated therefrom which by appropriate selection of manufacturing parameters can provide a predetermined upper temperature limit.
Another object of the present invention is to provide heating materials for and devices fabricated therefrom which are inexpensive to manufacture, safe to use and well adapted for their intended use.
Surprisingly, the above objectives can be realized and new compositions provided which overcome the problems associated with previous materials which have been used for the fabrication of microwave heating susceptors. The present compositions and devices do not exhibit runaway heating yet generate relatively large amounts of heat. Indeed, the final heating temperature can be controlled quite closely. Also, the present compositions are comprised of materials which are commonly available and cheap. In the most surprising aspect of the present invention, the compositions comprise ceramic materials previously considered to be microwave transparent or used in microwave transparent ceramic compositions.